The present invention relates to an implantable drug delivery system and to a method of administrating a drug using the drug delivery system.
1. Field of the Invention
Pharmaceutical drugs can be administrated to patients in many different ways, such as orally, nasally, through electroporation or by injections. Injections can be intravenous, intramusculaire or subcutaneous. The way of administration of drugs to a patient is often depending on properties of the drug, the organ to be treated and the patient""s physical and social presumptions to control the prescribed treatment. The efficiency of the prescribed drug treatment is also highly dependent on the patient""s abilities to follow the prescription. It is a well-known fact that the ability to follow a prescription is related to the patient""s understanding and knowledge of his disease and his possibility to follow the prescribed form of treatment.
Medical treatment by subcutaneous injections during a long period of time in patients that due to physical or social reasons themselves cannot make the injection are requiring assistance from e.g. a nurse or a relative.
The injection itself might have considerable side effects because a relatively high concentration of an active substance is administrated to one local site beneath the skin. Irritation of the skin or local haematoma is common for some patients.
Parameters that affect how frequent the injections are taken are the subcutaneaous half-life time for the drug, i.e. the time it takes for half the injected dose to be absorbed by tissue, the actual dose and of course the patient""s ability to follow the prescription.
The injected volume depends on the type of drug used and the wanted effect of the treatment and is normally between 0.2-0.6 ml per day given in 1-3 injections.
The costs to offer sterilized injection syringes and to discard them after use is a considerable part of the total cost for the treatment.
2. Description of the Prior Art
U.S. Pat. No. 4,193,397 discloses an implantable infusion apparatus and method. Although this reference mentions the possibility of having a battery powered electrical valve to perform the pumping action, a preferred pump embodiment employs a valve that can be actuated extracorporeally either manually or magnetically. Modifying the flow rates and volumes of the apparatus components, particularly the mixing chamber and the apparatus outlet tube, can provide a dosage profile tailored to the patient""s requirements. The described embodiment requires the patient""s contribution that, according to the discussion above, is a major drawback.
Also U.S. Pat. No. 5,769,823 discloses an implantable infusion pump. The purpose of this known device is primarily to provide an inexpensively manufacturable infusion pump. This is accomplished by arranging a plastic casing with a bellows that receives a propellant producing a vapour pressure that exerts a pressure on two resilient bags containing a medicament. When a throttle means is opened a clearly defined quantity of the medicament passes out of the bags into a catheter and into the body of a patient.
U.S. Pat. No. 4,673,391 discloses a micropump disposed within a human body for continuously delivering small quantities of a pharmaceutical liquid stored therein to be injected in a human body. The control part of the pump is placed outside the body and generates a magnetic force that can open a valve of the implanted micropump and release the pharmaceutical liquid. A problem with this known device is that it requires that the patient actively controls the device, which is not always is possible or desirable.
U.S. Pat. No. 5,752,930 relates to an implantable infusion pump for administration of a pharmaceutically active agent via a catheter to a plurality of infusion sites located at spaced intervals in a portion of a body to be treated by the agent. The catheter is provided with elution holes to perform the administration of the agent. However, experience has shown that the flow rate of the agent is not uniform through all elution holes. In particular, at low flow rates, fluid moving down the catheter exits the catheter at the elution hole having the least fluid resistance to flow to that relatively little or no fluid exits the catheter through the remaining elution holes. This results in overdose to some sites (the elution hole with the least fluid resistance) and underdose to other sites (remaining elution holes). The device described in U.S. Pat. No. 5,752,930 is directed to solve that problem. The solution is shortly to apply a first pressure to the agent in the catheter for a first time period and a second pressure to the agent for a second time period so that substantially the same dosage of agent can be applied to each of the sites.
One drawback with this solution is that it is technically rather complicated in that it requires an accurate control of the applied pressure.
One major drawback with implantable infusion pumps according to the prior art is that they cannot easily be optimized with regard to flow-rate of the drug and the volume of drug stored in the pump.
Another drawback with the prior art infusion pumps is that they have a complicated structure inter alia due to that they all administer the drug via a catheter.
It is an object of the present invention to provide an implantable drug delivery system where the above-mentioned drawbacks are avoided.
The drug is delivered to tissue through drug delivery openings that are arranged around the periphery of the dispenser module. At least two openings are provided.
In order to provide an easy-handled implantable infusion system the drug delivery openings are arranged around the periphery of the implant. This is advantageous in that no delivery catheter is needed according to a preferred embodiment of the invention.
By arranging several openings a most advantageous drug delivery is achieved in that only a limited volume is delivered at each location which is favorable in that the risk of negative effect on tissue as a result of too high drug exposure is almost eliminated.
An ideal situation exists if the limited volume delivered from one opening is in a one to one relationship with the absorption at the delivery site. In other word if the capillary force of the tissue at the delivery site provides for continuous absorption of all delivered drug at the delivery site then the ideal situation occurs.
Furthermore, according to still another embodiment of the invention, it is possible to only use some of the delivery openings and according to predetermined pattern xe2x80x9cactivatexe2x80x9d different openings at different times.
The drug supplying means comprises drug delivery channels, which is a system of channels for providing the delivery openings with drug. In an implantable drug delivery system it is of greatest importance to know the exact volume of the delivered drug dose.
However, in a system with a drug delivery channel having several openings of the same size arranged along a channel with the same inner diameter along the channel the pressure inside the channel changes along the channel. This pressure change results in a gradually decreasing delivered volume when approaching the distal end of the channel (the end most far from the drug reservoir). In order to arranged a system where the delivered volume is controllable and almost the same for all openings the area of the openings increases along the channel towards the distal end according to a predetermined relationship.
It is also possible to arrange a separate delivery channel for each delivery opening. In that case and if they have the same length then the area of the separate openings can be same resulting in that the same volume is delivered from each opening.
According to still another preferred embodiment of the present invention it makes it possible to tailor the treatment by choosing a reservoir module and a dispenser module having a desired performance for achieving an optimal treatment, i.e. the possibility to administer a prescribed dose during a prescribed time in a controlled manner.
The invention is especially useful for short-term implantation for administration of drugs during some weeks or month after a surgery, e.g. related to hip or knee prosthesis, when the patient is immobilized during the recovery period.